review of literatures on noble metal nanoparticles: properties...
TRANSCRIPT
Review of Literatures on
Noble Metal Nanoparticles:
Properties and Applications
Chapter 2
“Prepare for the unknown by studying how others in the past have coped with
the unforeseeable and the unpredictable.”
-George S. Patton
Chapter 2 Review of Literatures
Center for Interdisciplinary Research, D. Y. Patil University, Kolhapur 26
“Nanotechnology in medicine is going to have a major impact on the survival of the human race.” -Bernard Marcus
2.1. Introduction
The Unique optical property of the noble metal nanoparticles has made
them a potential candidate in the nanotechnology field. Zijlstra et al. have
demonstrated a DVD disk having storage capacity of 10 terabytes,
approximately 2000 movies of convectional size [1]. This is possible because
of the superior optical properties of randomly oriented gold nanorods
embedded in the disk. As the metal nanoparticles show size and shape
dependant optical properties, Zijlstra and coworkers used optical spectrum and
different polarization directions to store data. The enhancement in the optical
and photothermal properties of noble metal nanoparticles e.g., AgNPs, gold
nanoparticles, Pb nanoparticles etc., arises from resonant oscillation of their
free electrons in the presence of light, also known as localized surface plasmon
resonance (LSPR). The plasmon resonance can either radiate light (Mie
scattering), a process that finds great utility in optical and imaging fields, or be
rapidly converted to heat (absorption); the latter mechanism of dissipation has
opened up applications in several new areas.
So here, this chapter is divided into following parts. Some historical
evidences of use of AgNPs along with other noble metal nanoparticles are
given at first. The overview of various properties of AgNPs is described
further. Finally, we highlight some recent applications based on properties of
AgNPs.
2.2. Historical Perspective
In ancient time, silver was considered more precious than gold and
symbol of purity. Hippocrates proclaimed that silver contained medicinal
properties and could cure multiple maladies 10. Since silver was publicized to
have antiseptic properties, Phoenicians used silver vials for food storage to help
prevent spoilage. Ravelin (1869) and von Nageli (1893) had done pioneering
work in the study of anti-bacterial properties of silver [2]. Based on silver a
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Center for Interdisciplinary Research, D. Y. Patil University, Kolhapur 27
number of products commercialized throughout the 20th
century like Katadyn,
Movidyn, Argyrol, Alagon, Tetrasil, etc. [3]. Prior to the wide spread use of
antibiotics, during World War I silver compounds were used to help prevent
infection.
History tells about the use of these noble metals in stained glass to
produce the beautiful colors of drink cups, such as the Lycurgus cup [4], or
ancient windows, for example, in the Altenberg dome near Cologne in
Germany [5]. The early alchemist tried to produce portable gold, ‘‘the elixir of
life’’ as they considered gold as indestructible and have enormous medicinal
importance [6]. The German bacteriologist Robert Koch discovered in 1890
that low concentrations of potassium gold cyanide, K[Au(CN)2] had
antimicrobial activity against the tubercle bacillus. From thereafter gold is
introduced in modern medicine [7]. During the Vedic period (1000BC-600BC)
in ancient India, the use of gold, in the form of swarna bhasma (meaning, gold
ash), is suggested for medicinal purposes started [8].
In1856, Faraday had hypothesized that the color of ruby glass and its
aqueous solutions of gold (mixed with either SO3 or phosphorus), is due to
finely divided gold particles [9]. Stoke had different opinion about the
existence of a purple gold oxide, apparently prompted Faraday’s famous
electrical discharge method for preparing aqueous gold colloids [10]. It is
noteworthy that Faraday did not have a quantitative theoretical framework, but
seems to have based his postulate on an intuitive understanding of highly
reflective metals and scattering processes.
Gustav Mie’s 1908 paper represents the first rigorous theoretical
treatment of the optical properties of spherical metal particles [11, 12]. Mie’s
theory yielded extinction coefficients for nanoscopic gold particles which
compared well with the experimental spectra of gold sols and unlike Maxwell-
Garnett theory, was applicable to spheres of any size. Mie scattering theory is
applied today to a variety of systems, including nonmetal particles. His basic
approach has also been adapted to other shapes, such as cylinders [13] and
ellipsoids [14].
Chapter 2 Review of Literatures
Center for Interdisciplinary Research, D. Y. Patil University, Kolhapur 28
In 1727, a German professor, John Herman Schulze first revealed that
on exposure to light silver salts turned into black. Silver salts then after were
further explored and were for many years the critical component of
photographic plate. Platinum was discovered on the alluvial sands of the Pinto
River in Columbia and confirmed as new metal by Italian-French scientist
Julius Caesar Scaliger who concluded that the metal was not silver and in fact a
new element [15]. In 1845, Michel Peyrone synthesized cisplatin (platinum-
containing anti-cancer drugs) [16] and Alfred Werner, in 1893, elucidated the
structure of cisplatin whereas Rosenberg studied the antitumor activity of
cisplatin. After successful studies and trials, in 1978 the US by the Food and
Drug Administration approved the use of cisplatin as anticancer drug.
2.3. Properties of Noble Metal Nanoparticles
2.3.1. Optical Properties
Metal nanoparticles exhibit unique shape and size-dependent optical
properties. In particular, the extinction spectra of noble metal nanoparticles are
dominated by localized surface plasmon resonance (LSPR). These resonances
are attributed to a coherent excitation of the conduction band electrons, excited
by means of an electromagnetic field. In the presence of the oscillating
electromagnetic field of the light, the free electrons of the metal nanoparticle
undergo a collective coherent oscillation with respect to the positive metallic
lattice. The general case of an interaction between an electromagnetic field and
a spherical metal nanoparticle is schematically depicted in figure 2.1. The field
in the nanoparticle is in homogeneous and its optical response is rather
complex [17].
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Figure 2.1 Schematic of plasmon oscillation for a sphere, showing the
displacement of the conduction electron charge cloud relative to
the nuclei [15]
Large number of atoms present on the surface of nanoparticles
contributes electron cloud on the surface of nanoparticles. As shown in the
figure 2.1, the movement of these free electrons under the influence of the
electric field vector of the incoming light leads to a dipole excitation across the
particle. This induces positive polarization charge on cationic lattice. This
charge acts as a restoring force, and brings back electron cloud to its original
position, thus causing the oscillations of the electron cloud. In this manner a
dipolar oscillation of electrons is created (called plasma oscillation) with a
certain frequency called plasmon frequency. The surface plasmon resonance is
a collective excitation mode of the plasma localized near the surface. However,
the resonance frequency of the surface plasmon is different from an ordinary
plasma frequency. If the frequency of the excitation light field is in resonance
with the frequency of this collective oscillation, even a small exciting field
leads to a strong oscillation [18, 19].
The rigorous calculations made by Mie are traditionally referred to as
the Mie theory. The theory of light scattering by small spheres was already
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developed, but it was Mie who applied it to the scattering and absorption of
metal spheres. Mie theory describes characteristic light absorption of spherical
metal nanoparticles which demonstrates that the absorption does not strongly
depend on the particle size in the size range of 3 to 10 nm. The particles start
showing size dependence of the plasmon resonance band below the size limit
of 10 nm and ultimately disappear for particles of size less than 2 nm. This is
attributed to the decreasing validity of the free electron gas model assumption
[20, 21].
The electric field intensity and the scattering and absorption cross-
sections are all strongly enhanced at the LSPR frequency [22] which for gold,
silver, and copper lies in the visible region [22, 23]. Since copper is easily
oxidized, gold and silver nanostructures are most attractive for optical
applications. The ability to integrate metal nanoparticles into biological
systems has had greatest impact in biology and biomedicine. Due to the
plasmonic properties of gold and silver nanostructures, they are being utilized
for biodiagnostics, biophysical studies, and medical therapy.
2.3.2. Catalytic properties
Metal nanoparticles like silver, palladium and gold have attracted a great
interest in scientific research and industrial applications, due to their unique
large surface-to-volume ratios. Catalytic activity of usually work on the surface
of metals, the metal nanoparticles have been considered as promising materials
for catalysis, which have much larger surface area per unit volume or weight of
metal than the bulk metal. The size of the nanoparticle is one of the most
important factors in the catalytic activity. As surface-to-volume ration increases
the catalytic activity of the metal nanoparticles increases which ultimately
increases with decrease in the size of the nanoparticles. Pd and Ag
nanoparticles can be used for the hydrogenation of oxygen-containing olefins
and epoxidation of ethylene with oxygen, respectively [24]. Thus, metal
nanoparticles have been demonstrated for homogeneous and heterogeneous
catalytic activity. The catalytic activity is also shape dependent phenomenon as
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the shape of the nanoparticle changes the surface-to-volume ration also changes
which causes the exchange of electrons. The hydration of acrylonitrile was
catalyzed by Cu-Pd bimetallic nanoparticles [25] and like that plenty of
bimetallic have been studied for the catalytic activity.
2.3.3. Electronic properties
Metal nanoparticles show different electronic properties below certain
size limit. There will be change in electronic distribution in metal which may
transform some metal into semiconductor at nanometer scale. For example,
gold nanoparticles act as quantum dot below certain size range [26]. Coulomb
blockade is important and interesting effect that occurs when nanoparticles
embedded between metal-insulator-metal junctions show charging of
differential capacitance or charging at low temperature even at zero bias. This
effect is a result of extremely small capacitance of the metal nanoparticles [27].
2.3.4. Melting point
Many physical properties of materials, especially the melting point,
change as the physical size of the material approaches from the micro to
nanoscale [28]. Changes in melting point occur due to a much larger surface-
to-volume ratio of nanoscale material than bulk materials, altering
thermodynamic and thermal properties. The melting temperature also decreases
as the metal particle size decreases. The melting point is also marginally
depending on shape of the metal nanoparticles. The melting point decrease with
the large surface-to-volume ratio. The surface areas of nanoparticles in
different shape will be different even in the identical volume, and the area
difference is large especially in small particle size.
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2.4. Applications of Noble Metal Nanoparticles
Figure 2.2. The schematic of the various applications of noble metal
nanoparticles in the therapeutics, imaging, diagnosis
Metal nanoparticles due to the varies physical, chemical and biological
properties also size and shape dependent phenomenon have potential
application in vast area like antimicrobial agents, therapeutic, imaging,
diagnostic, catalysis, environmental decontamination etc., (figure 2.2). The
following section will briefly cover the various metal nanoparticles used in
various applications.
2.4.1. Antimicrobial properties
Among noble metal nanoparticles, silver, copper and gold have been
studied extensively for antimicrobial activity against pathogenic
microorganisms [29]. High surface areas to volume ratios with unique
physicochemical properties of noble metal nanoparticles are contributing
effective antimicrobial activities [30]. Lots of burden on the economy for the
developing countries is rising due to the production cost of traditional
antibiotics [31]. Also, microorganisms are developing resistance against these
antibiotics. So continues development in the traditional antibiotic is required
desperately. The biological synthesis method of metal and metal oxide
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nanoparticles have been explored since few years. So the production cost of
these materials is too less than chemical antibiotics. It is also demonstrated that
the rate of acquiring resistance to the nanoparticles is slower as compared to
conventional antibiotics [32]. Antimicrobial mechanisms of nanomaterials
include: 1) photocatalytic production of reactive oxygen species (ROS) that
damage cellular and viral components, 2) damaging the bacterial cell
wall/membrane, 3) disruption of energy transduction, and 4) inhibition of
enzyme activity and DNA synthesis [30, 33-35]. Figure 2.3 shows the brief
mechanisms of antimicrobial activity of metal nanoparticles.
Figure 2.3. Schematics of antimicrobial mechanisms of AgNPs [39]
The antibacterial property of silver has been used since antiquity. Silver
in the forms of metallic silver, silver sulfadiazine, and silver nitrate has been
used for burn wound treatment and disinfection of catheters, dental
instruments, and bacterial infection control [36]. Using after discovery of
antibiotics in the 1940s silver become out of favor to treat bacterial infections
[37]. The clinical use of silver is now under extensive research as antibiotics-
resistant bacteria are emerging and showing less or no response to conventional
antibiotics [38]. AgNPs among other metallic NPs have proven to be the most
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lethal against viruses, bacteria, and other eukaryotic microorganisms [39]. The
respiratory chain and cell division are affected by AgNPs which finally lead to
cell death [40]. The antimicrobial activity of AgNPs is oppositely allied to size
and shape [41]. Amalgamation of AgNPs with antibiotics such as penicillin G,
erythromycin, amoxicillin, and vancomycin, resulted in improved and
synergistic antimicrobial effects against Gram-positive and Gram-negative
bacteria [42, 43]. AgNPs have been used in fabrics, textiles, surgical masks and
medical devices, gels and lotions [44]. Irreversible pigmentation in the skin
(argyria) and eyes (argyrosis) with also other toxic effects like irritations, organ
damage, and blood count change are the possible adverse effects due to
prolonged exposure of Ag+ ions [45]. On the other hand, metallic silver appears
to have a minimal risk to health and AgNPs are recommended to be non-toxic
in some studies [46], but concentration-dependent adverse effects of AgNPs on
the mitochondrial activity are also report. So AgNPs to be used as the sole
antimicrobial agent against diseases is required to undergo thorough study of
effects to mammalian and other animal cells.
Near-infrared (NIR) light-absorbing AuNPs, nanoshells, nanorods, and
nanocages have been demonstrated to kill bacterial growth via irradiation with
focused laser pulses of suitable wavelengths [47]. AuNPs have strong attraction
towards negatively charged cell membrane of microorganisms which is the
main cause of the antimicrobial activity of the AuNPs [48], which was also
supported by the observation that cationic particles were found to be slightly
toxic while anionic particles were not. AuNPs conjugated with antimicrobial
agents and antibodies have been investigated to obtain selective antimicrobial
effects. For example, Au NPs conjugated with anti-proteinA antibodies for the
selective killing of S. aureus, which target the bacterial cell surface, was
reported [49]. Laser induced hyperthermia caused formation of the bubble
which is used for the selective killing of the bacteria [50]. Effective inhibition
of antibiotic resistant microorganisms with the conjugation of antibiotic is
demonstrated [51]. Therefore, AuNPs can be promising adjuvants for antibiotic
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therapy in treating serious bacterial infections with minimal adverse effects as
AuNPs have been demonstrated to be less toxic to mammalian cells.
2.4.2. In therapeutics
Versatile noble metal nanoparticles are agents with a variety of
biomedical applications and sensitive diagnostic assays [50], radiotherapy
enhancement, and thermal ablation [51, 52], as well as gene delivery and drug
[53, 54]. In addition, noble metals NPs have been projected as non toxic
carriers for drug and gene-delivery applications [55]. Theranostic application is
the major advantage of noble metal nanoparticles where they are being
explored to use as simultaneous diagnostic as well as therapeutics purpose.
It is necessary to use designed and engineered NPs especially in therapy
that can be targeted to tissues of interest, also can produce specific, desired
effects. The function of AgNPs as antibacterial agents has been well
established. Anti-viral properties of biologically formed AgNPs are more
effective than chemically synthesized AgNPs [56]. Vero cells co-incubated
with AgNPs were reported to prevent plaque formation after being infected
with the Monkey pox virus [57] Metallic nanoparticles have also been
described as a possible HIV preventative therapeutic [58]. Metallic
nanoparticles have also been effective antiviral agents against herpes simplex
virus, influenza, and respiratory syncytial viruses [59]. The potential
therapeutic application of noble metal nanoparticles represents an attractive
platform for cancer therapy in a wide variety of targets and clinical settings. It
is shown that ‘‘naked’’ gold nanoparticles inhibited the activity of heparin-
binding proteins, such as VEGF1and bFGF in vitro and VEGF induced
angiogenesis in vivo [60]. Dalton’s lymphoma ascites (DLA) cell lines co-
incubated with AgNPs displayed a dose dependent toxicity through activation
of caspase-3 and inhibition of cellular proliferation. Furthermore, tumor
bearing mice injected with AgNPs demonstrated a reduction of ascites
production (65%) and tumor progression compared to the sham treated mice
[61] which have been shown to have applications in cancer treatment. Human
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Center for Interdisciplinary Research, D. Y. Patil University, Kolhapur 36
colon carcinoma cells (HT29) showed a dose and time dependent response
when exposed to platinum nanoparticles (Pt-NP) [62].
Gold nanoparticles have shown potential as intracellular delivery
vehicles for antisense oligonucleotides [63] and for therapeutic siRNA by
providing protection against RNAses and ease of functionalization for selective
targeting [64] have emerged as a powerful and useful tools to block gene
function and for sequence-specific posttranscriptional gene silencing, playing
an important role in down regulation of specific gene expression in cancer
cells. Metallic nanoparticles may offer an advantage in radiotherapy area by
utilizing their excellent optical properties, surface resonance, and wavelength
tunability. For example X-ray irradiation, gold nanoparticles can induce
cellular apoptosis through the generation of radicals which can be used as
therapy without harming the surrounding healthy cells [65]. The surface size of
AgNPs enhances the thermal sensitivity of glioma cells. Pioneering work by
Pitsillides illustrated that the surface plasmon resonance (SPR) of nanoparticles
is easily exploited for photo-dynamic therapy (PDT) anti-cancer therapy [66].
El-Sayed group demonstrated that AuNPs are effective PDT agents and could
‘‘seek and destroy’’ cancerous cells [67].
The metallic nanoparticles have been demonstrated promising as PDT
and hyperthermic agents for the treatment of cancer. Rapid heating of metallic
nanoparticles occurs on the application of magnetic field. The gold particles by
the oscillating magnetic field produce electric currents [68]. The heat
generation occurs due to eddy which dissipates into the surrounding from
nanoparticles, causing thermal ablation [69]. Figure 2.4 is demonstrating the
treatment of cancer using metallic nanoparticles by inducing hyperthermia with
the aid of radiotherapy. The toxicity of metallic nanoparticles to the normal
tissue or cells is the major concern of the use of metallic nanoparticles for the
use of cancer hyperthermia.
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Figure 2.4. Therapeutic application of noble metal nanoparticles by the
induction of hyperthermia with the assists of radiotherapy [64]
2.4.3. In diagnostics and imaging
Due to the strongly enhanced LSPR, noble metal nanoparticles scatter
light very strongly at the LSPR frequency, making them very promising for
optical imaging and labeling of biological systems [49]. While scattering and
absorption are competing processes, the relative contribution of scattering
increases rapidly with increase in the nanostructure volume. Dark-field
microscopy shows (right) HSC cancer cells clearly defined by the strong LSPR
scattering from (top) gold nanospheres and (bottom) gold nanorods bound
specifically to the cancer cell surface, whereas (left) HaCat healthy cells have
(top) gold nanospheres and (bottom) gold nanorods randomly dispersed
without specific binding (figure 2.5). The scattering color (LSPR frequency) of
the nanospheres and nanorods can be clearly distinguished. While the
scattering from a 10-nm gold nanoparticle is negligible, an 80-nm gold
nanoparticle offers scattering 5 orders of magnitude larger than the typical
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emission from a dye. Such highly enhanced cross-sections offer sensitive and
highly contrasted imaging allowing use of the much simpler but powerful dark-
field microscopy [64].
Figure 2.5. Molecular-specific imaging of cancer using metal nanoparticle /
anti-EGFR conjugates [21]
The noble metal nanoparticles can be used for the simultaneous actuation and
tracking in vivo along with their therapeutic capabilities. Most noble metal
nanoparticles for in vivo imaging and therapy have been designed to strongly
absorb in the near infrared (NIR) so as to be used as effective contrast agents
whereas at NIR biological components minimum light absorption [46]. The use
of PEG-coated AuNPs for in vivo computed tomography CT contrast agent was
shown to increase image contrast, which allows to reduce the radiation dosage
needed, allow to overcome the limitations of conventional contrast agents (e.g.,
iodine-based compounds), such as short imaging times due to rapid renal
clearance, renal toxicity, and vascular permeation [46]. In photoacoustic
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imaging (PAI) and photoacoustic tomography (PAT), Au-nanocages enhanced
the contrast between blood and the surrounding tissues by up to 81%, allowing
a more detailed image of vascular structures at greater depths [69].
2.4.4. In catalysis and environmental decontamination
Noble metals and bimetallic nanoparticles have shown their potential
use for the environmental application in conversion of hazardous chemical
compound to less toxic or non-toxic one. Liu et al. reported
hydrodechlorination of monochlorobenzene in the presence of colloidal
platinum nanoparticles stabilized on polyvinylpyrrolidone (PVP) [70].
Silver/gold (Ag/Au) bimetallic nanoparticles have also been reported as being
effective for dehalogenation processes. Using an underpotential deposition-
redox placement technique, Zhou used Ag/Au bimetallic nanoparticles as the
cathode in the electrochemical reduction of benzyl chloride [70].
Platinum/palladium (Pt/Pd) bimetallic nanoparticles, iron/silver (Fe-Ag) and
gold/platinum (Au-Pt) are increasingly being investigated for dehalogenation
and it is expected that with optimization of reaction conditions [70].
A number of contaminants such as lead and pesticides are affecting
water supplies globally due to their widespread use; pollutants of geochemical
origin such as arsenic and fluoride are currently found in selected countries. It
is quite evident that pesticides contain highly toxic recalcitrant groups and
hence are extremely difficult to break through normal synthetic routes of
degradation. A useful example of size-dependent catalysis has been
demonstrated through AgNPs-catalyzed reduction of nitrophenol to
aminophenol [66]. Contaminants such as pesticides are removed through
homogeneous or heterogeneous chemistry. A remarkable example is the
catalytic oxidation of CO with supported gold nanoparticles [71]. One of the
interesting application areas of noble metal nanoparticles in drinking water
purification is the sequestration of heavy metals. The first studies of the
interaction of metal ions with noble metal nanoparticles were in the early 1990s
[71]. Noble metal nanoparticles have extensively studied for the detection of
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heavy metals and pesticides in the drinking water [72]. In addition, the biggest
class of contaminants affecting drinking water is microorganisms. Every year,
1.8 million people die from diarrheal diseases (including cholera); 90% are
children under five, mostly in developing countries. Worldwide, 1 billion
people lack access to safe drinking water, 2.4 billion to adequate sanitation.
Improved water supply reduces diarrhea morbidity by 6% to 25%; improved
sanitation reduces it by 32%. As discussed earlier, the antimicrobial activity of
the AgNPs has been top priority of the researcher which is now also being used
for the decontamination of water from pathogenic species [72].
2.5. Summary
The present chapter deals with some historical and theoretical
background of the noble metal nanoparticles. Due to their unique size and
shape dependent optical properties, metal nanoparticles have potential
applications in medicine and environmental field. This chapter gives some
glimpse of the properties, and applications of the noble metal nanoparticles in
various fields including health care and environmental decontamination.
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Center for Interdisciplinary Research, D. Y. Patil University, Kolhapur 41
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